Voltage control of an ac drive system during motor starting

ABSTRACT

Method and apparatus for providing rapid synchronization of one or more synchronous motors being started in a system in which a source of adjustable voltage a-c electric power such as an inverter supplies electric power to the motors being started and to a plurality of previously operating motors. A selected output parameter of the source which changes substantially from its steady state level during the starting of one or more motors is monitored, and the output voltage of the source is increased for a predetermined period following the sensing of a change in the output parameter which is consistent with the starting of one or more motors, the increased output voltage implementing rapid synchronization of the motors being started without changing the operating speed of the previously operating motors.

United States Patent Pedersen Sept. 4, 1973 VOLTAGE CONTROL OF AN A.C.DRIVE SYSTEM DURING MOTOR STARTING [75] Inventor: Niels Peder Pedersen,Erie, Pa. ABSTRACT [73] Asslgnee: General Electric Company Method andapparatus for providing rapid synchroniza- [22] Filed: Oct. 24, 1972tion of one or more synchronous motors being started pp No: 299,756 In asystem 1n whlch a source of ad ustable voltage ac electnc power such asan mverter supplles electnc power to the motors being started and to aplurality of [52] U.S. Cl 318/85, 3l8/7l, 3l8/l0l previously operatingmotors. A selected output param- [51] Int. Cl. I-I02p 5/46 eter of thesource which changes substantially from its [58] Field of Search 318/66,71, 85, 101, steady state level during the starting of one or more318/102, 103 motors is monitored, and the output voltage of the sourceis increased for a predetermined period follow- [56] References Citeding the sensing of a change in the output parameter UNITED STATESPATENTS which is consistent with the starting of one or more mo-3,205,420 9/1965 Cobb 318/102 x lncleased "wage P 3 551 775 12 1970Safiuddin 3l8/7l x symhmmzam" of the motors bemg Started 3,663,8755/1972 Ashiya 318/71 x changing the Operating Speed Ofthe Previously ping motors.

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SHEET 1 0P3 I04 100 4. K 38 f VOLTAGE SE ADJUSTING CHANGE g 'QNURPQQQTSIGNAL SENSOR SENSO GENERATOR R 24. 20 I40. K Z 2 COMMAND r REGULATINGPOWER T SOURCE SYSTEM SOURCE FIGJ PAIENTEU 8E7 4513 SHEET- 3 0F 3 l I Il I l l FIG.4

VOLTAGE CONTROL OF AN A.C. DRIVE SYSTEM DURING MOTOR STARTING BACKGROUNDOF THE INVENTION 1. Field of the Invention This invention relates to a-cdrive systems of the type in which a plurality of synchronous motors aresupplied with electric power from a single source and, moreparticularly, to the control of such drives in a manner such that one ormore motors may be rapidly synchronized without changing the speed ofpreviously synchronized motors.

2. Description of the Prior Art It is often desirable that the motor ofan electric drive system operate at a precisely determined speedcontinuously without speed fluxuations. This is particularly true incase of certain process applications of drive systems. In addition, itis sometimes desirable that motors not only operate with extreme speedaccuracy, but also that a number of motors be supplied with electricpower from a single source. By utilizing motors of the synchronous type,such as synchronous-reluctance motors, an entire bank of motors can bemade to operate at either identical speeds or at speeds related to eachother in known ratios. By utilizing an adjustable frequency, adjustablevoltage source such as an inverter or cycloconverter, the operatingspeeds of synchronous motors can be adjusted to provide the preciseoperating speeds required by the drive application. Where this type ofpower source is used, heating of the motors during operation can becomea significant problem due to harmonic content in the electric powersupplied to the motors. This heating can be somewhat alleviated bymaintaining the output voltage of the source at the lowest levelconsistent with operation of the motors.

In drive systems including a plurality of synchronous motors, it issometimes desirable to be able to start and stop the motorsindependently of each other. It is particularly desirable to be able tostop and restart one or more motors while the remaining motors continueto operate at their synchronous speeds. Where, however, motor heatinghas occurred prior to a stop, it may be difficult or even impossible torestart the motor while it is still hot due to the reduced torquegenerated by a hot motor. One apparent way to approach this problemwould be to permanently increase the normal output voltage of the sourceso as to produce the necessary torque for starting purposes. Thisapproach is not, however, entirely satisfactory in that the result wouldbe still more heating in all motors and, consequently, a requirement fora still higher voltage. In the past, it has been suggested that theoutput voltage be increased only for the period of time required forstarting and synchronizing the motors and that the output voltagethereafter be returned to a lower steady state level. In this way, a hotmotor can be reliably started without subjecting all of the motors toexcessive steady state heating. This approach has heretofore beenimplemented by providing an interlock with the starting means for eachmotor or each group of motors, the interlock operating through thecontrol apparatus of the source to increase the output voltage for apredetermined period. In drive systems incorporating a relatively smallnumber of motors, this approach is generally satisfactory. However, thisapproach has been found not to be entirely satisfactory for drivesystems having a large number of independently operable motors, e.g.,synthetic fiber process applications in which 64 motors are typicallyconnected to a single inverter. In such a case, as many as 64 interlocksand control connections could be required, thus introducing substantialexpense and complexity into the system.

SUMMARY OF THE INVENTION Accordingly, it is a primary object of thisinvention to provide improved means for assuring rapid and reliablestarting of synchronous motors in a drive system in which the motorsbeing started and a number of previously synchronized motors areconnected to a source of adjustable voltage a-c electric power.

Another object of this invention is to assure rapid and reliablerestarting and resynchronization of motors in a drive system in whichthe motors being started and previously synchronized motors areconnected to a source of adjustable frequency, adjustable voltageelectric power.

Yet another object of this invention is to provide the foregoing objectswithout changing the operating speeds of the previously synchronizedmotors.

Still another object is to provide the foregoing objects withoutrequiring the use of expensive and complex control circuitry including,wihout being limited to, interlocks associated with the motors adaptedfor independent starting and stopping.

Briefly stated, in carrying out the invention in one form, an a-c drivesystem including a source of adjustable voltage a-c electric power and aplurality of synchronous motors coupled to the source for receivingelectric power therefrom is provided with control means for facilitatingrapid and reliable starting of one or more of the motors. The controlmeans includes first means coupled to the electric power source forsensing an output parameter thereof which varies substantially from itssteady state level during the starting of one or more motors, the firstmeans producing a first signal having a magnitude representative of theselected output parameter. The first control signal is supplied tosecond means which produces a second control signal in response to asubstantial change in the first control signal, the second controlsignal being supplied in turn to third means for producing in responseto a second control signal a third control signal having a predeterminedduration following the initial reception of the second control signal.The third control signal is supplied to fourth means for increasing theoutput voltage of the a-c electric power source throughout thepredetermined duration of the third control signal. The in- .creasedvoltage permits rapid starting and synchronization of the motors beingstarted, but the limited duration of the voltage increase limits theheating effect of the voltage increase on the motors.

. By a further aspect of the invention, the output parameter sensed bythe first means is the component of output-current that is in phase withthe output voltage of the source. By a still further aspect of theinvention, the frequency of the a-c source, which may be an in verter,cycloconverter, or other static power converter, is not changed inresponse to the third control signal. In this manner, the motorsoperating and synchronized prior to the starting process continue tooperate at their synchronous speeds without any adverse speedfluxuations. By still further aspects of the invention, the second meansis responsive to only increases in the first control signal and thethird means is responsive to seend control signals to produce thirdcontrol signals having substantially constant magnitudes throughout thepredetermined duration and magnitudes established independently of themagnitude of the second control signals.

BRIEF DESCRIPTION OF THE DRAWINGS While the novel features of thisinvention are set forth with particularity in the appended claims, thinvention, both as to organization and content, will be betterunderstood and appreciated, along with other objects and featuresthereof, from the following detailed description taken in conjunctionwith the drawings, in which:

FIG. 1 is a block diagram of an a-c drive system including incorporatingthe invention;

FIG. 2 is a circuit diagram of a preferred embodiment of the outputparameter sensor of FIG. 1, the illustrated embodiment comprising anin-phase current sensor suitable for use in a threephase drive system;

FIG. 3 is a circuit diagram of the change sensorand of one embodiment ofthe voltage adjusting signal generator of FIG. 1; and

FIG. 4 is a circuit diagram of a second embodiment of the voltageadjusting signal generator.

DETAILED DESCRIPTION A drive system incorporating the present inventionis illustrated by FIG. 1, the drive system including a source ofadjustable voltage electric power. The power source 10 is preferably astatic inverter for converting electric power from a d-c source 12 to3-phase adjustable frequency, adjustable voltage electric power onoutput phase conductors 14A, 14B and 14C. The source 10 may, however,take on other forms such as a cycloconverter for converting a-c electricpower to adjustable frequency, adjustable voltage electric power. Thesource 10 may supply single phase rather than 3-phase electric power.

The output conductors 14A, 14B and- 14C are connected through individual3-phase conductors 16A through 16H (shown in single line form) andrespective individually operative starting and stopping contacts 18Athrough 18H to a respective plurality of 3-phase synchronous motors Mlthrough M8. While only eight motors are illustrated, it is typical for asingle source 10 to supply even greater numbers of motors. For example,in the synthetic fiber industry, a single power source often supplies asmany as 64 motors which are capable of being individually started andstopped. Thus, the eight motors Ml through M8 are merely representativeof a plurality of motors coupled to a single source.

Let it now be assumed that the drive system is operating with contacts18A through 180 closed as illustrated such that motors Ml through M7 areoperating at their synchronous speeds. Let it also be assumed that aregulating system 20 is controlling the output frequency of the sourceor inverter 10 such that the synchronous speeds of the motors M 1through M7 are established in accordance with the magnitudes of acommand signal 22 from a command source 24 and appropriate feedbacksignals 26 forming no part of the present invention. The output voltageof the inverter 10 is also controlled by the regulating system 20 inresponse to the signals 22 and 26 and, under starting conditions, by asignal 28 generated in accordance with this invention.

If the contacts 18H are now closed, electric power at the outputfrequency and voltage of the inverter 10 will be applied to the motor M8to start the motor M8. During starting, it has been found that theoutput current of the source 10 rises very substantially as the resultof the starting of just one motor in a bank of motors in which a numberof motors are already running. For example, in a typical drive system inwhich 63 motors are operating synchronously, it has been found that thestarting of the 64th motor will momentarily increase the output currentfrom a steady state 189 amperes to 210 amperes. Thus, the closing of thecontacts 18H to start the motor M8 will cause a very significant rise inthe inverter output current on conductors 14. Similarly, the closing ofone of the contacts 18A through to start the respective one of themotors Ml through M7 while a number of the other motors are operatingwill result in a similar momentary increase in output current. v

In accordance with the present invention, the output current of theinverter 10 is monitored, and means are provided which are responsive toan increase in current consistent with the start of a motor to producethe voltage increasing signal 28. The signal 28 and the resultingincreases in inverter output voltage and motor torque persist for aperiod of time sufficient to bring the motor being started up to itssynchronous speed. The signal 28 is then automatically removed, and theoutput voltage drops to a steady state level sufficient for motoroperation withminimum heating.

Still referring to FIG. 1, the output current of each phase of theinverter 10 is sensed by means of current transformers (represented bynumeral 30 of FIG. 1) from which signals proportional to the actualcurrents in the conductors 14A, 14B and 14C are supplied over lines 32to an in-phase current sensor 34. Over lines 36, the in-ph'ase currentsensor 34 is supplied from the regulating system 20 with signalsin-phase with the output voltage of the inverter 10, the in-phasecurrent sensor 34 producing signal 38 proportional to the component ofthe output current that is in phase with the output voltage. With all ora significant number of the motors Ml-M8 running under steady stateconditions, the inphase current signal 38 will have a substantiallyfixed level. During subsequent start-up of one or more motors, thein-phase signal 38 will increase substantially. It has been found thatthe in-phase current component is an extremely responsive indicator of amotor start. Other output parameters of the inverter which changesubstantially with the starting of a motor can, of course, be used as anindication of motor starting. For example, the actual current as well asits in-phase component is a highly satisfactory output parameter forindicating the starting of a motor.

Referring now to FIGS. 1 and 2, the in-phase current sensing apparatuswill be described in greater detail. Current signals proportional to theactual phase currents in conductors 14A, 14B and 14C are generated,respectively, by current transformers 30A, 30B and 30C and are suppliedover conductors 32A, 32B and 32C to the in-phase current sensor 34. Theconductor 32A supplies the signal proportional to the actual phasecurrent in conductor 14A to a field effect transistor (FET) 40 at eitherits source or drain electrode. Since the field effect transistor 40 isbi-directional with respect to its current path through the source-drainelectrodes, the conductor 32A may be connected to either of the two, anda conductor 42 to a summing amplifier 44 is connected to the otherelectrode. The conductor 36A supplies a rectangular or square wavevoltage signal in phase with the voltage applied to conductor 14A by thepower source 10. As shown by FIG. 2, the voltage signal on conductor 36Ais coupled to the base electrode 46 of a conventional NPN typetransistor 48 by a resistor 50. A resistor 52 is connected to the gateelectrode 46 of the transistor 48 as well as to a ground or common point54 to stabilize the voltage level of the gate electrode when no voltagesignal is present on the conductor 36A. The collector electrode of thetransistor 48 is coupled through a resistor 56 to a positive biasvoltage and through a diode rectifier 58 to the gate of PET 40 andthrough yet another resistor 60 to the ground or common point 54. Thegate of the FET 40 is also connected through a resistor 62 to the commonpoint 54. The conductors 32B and 32C supply signals proportional to theactual phase currents in conductors 14B and 14C to similar circuits inwhich similar elements are identified by primed and double primedreference numerals, and square wave signals in phase with the outputvoltages of conductors 14B and 14C are supplied on conductors 36B and36C, respectively. Conductors 42' and 42" are also connected to thesumming amplifiers 44.

FIG. 2 also shows the summing amplifier 44 in detail. An input lead 64to the summing amplifier 44 serves to sum the phase current signals onconductors 42, 42 and 42" and to introduce the summation thereof into aninverting input 66 of an operational amplifier 68. A non-inverting input70 of the amplifier 44 is coupled by a resistor 72 to ground or commonpoint 54. In parallel with the amplifier 44 between the input lead 64thereto and its output conductor 38' are a gain-setting resistor 76 anda filter capacitor 78.

The operation of the in-phase current sensing apparatus 34 will now bedescribed. As previously explained, an electrical signal continuouslyproportional to the actual current in conductor 14A is supplied to theFET 40 by conductor 32A, and a square wave electrical signal in phasewith the voltage applied to conductor 14A is supplied on conductor 36Afrom the regulating system 20. As long as the transistor 48 is turnedoff, the positive voltage applied to the resistor 56 is sufficient tomaintain the FET 40 in a non-conductive state by applying a positivevoltage to its base 40'. As long as the FET 40 is non-conductive, therewill be an absence of a signal on conductor 42 to the summing amplifier44.

When, however, the voltage signal applied on conductor 36A changes fromthe relatively negative halfcycle to its relatively positive half-cycle,the positive voltage at the base 46 will be sufficient to turn on the.

transistor 48, thereby causing the voltage at junction 80 to drop toapproximately that of common point 54. As a result, the voltage at thebase 40 of the FET 40 drops sufficiently to turn on the FET 40, and thecurrent signal supplied on conductor 32A is passed to the conductor 42and the summing amplifier 44 by the FET 40. The FET 40 will remainconductive so long as the voltage signal supplied on conductor 36A isrelatively positive. When it drops to its relatively negative level atthe end of the positive half-cycle, the transistor 48 will immediatelyturn off and thereby turn off the FET 40. It will thus be appreciatedthat the actual current signal on conductor 32A is conducted to thesumming amplitier 44 during the full positive half-cycles of the voltagesignal, but not during the negative half-cycle.

Since the motors M1-M8 are inductive loads, the actual current in phaseA will not be in phase with the voltage of phase A. Accordingly, thesignal supplied to conductor 42 by the bi-directional FET 40 during thepositive half-cycle of voltage will be negative for a portion of theconductive time and positive during a portion of the conductive time. Ata given instant, the phase control signal to conductor 42, whether it isnegative or positive, will be proportional to the actual current inphase A at that instant. Over the entire positive half-cycle of voltage,the phase control signal will have an average magnitude proportional tothe average magnitude of the component of actual phase current which isin phase with the voltage. As long as the actual phase voltage andcurrent are symmetrical and attain equal positive and negative levels,the average magnitude of the phase control signal over a full cycle willbe proportional to the average magnitude of the in-phase component ofactual current.

The circuits connected to input conductors 32B and 36B and to inputconductors 32C and 36C operate in a similar manner to produce onconductors 42' and 42" electrical control signals proportional to theinphase components of current in conductors 14A and 14B. These signalsare summed at conductor 64 and are thus supplied to the invertingamplifier 68 of the summing amplifier 44 as an electrical signalproportional to the average of the three in-phase components of actualcurrent. Since at least one phase will have positive voltage at alltimes, the signal supplied to the amplifier 68 will be continuous eventhough signals will be present on any one of the conductors 42, 42' and42" only half of the time.

As indicated previously, the amplifier 68 inverts and amplifies thesignal on conductor 64 to generate a control signal on conductor 38, thecontrol signal being proportional to the average of the in-phasecomponents of current in phases A, B and C. The control signal isidentified in FIG. 1 by the numeral 38.

For a more complete explanation of the in-phase current sensingapparatus just described, attention is directed to copending patentapplication Serial No. 147,771, entitled Adjustable Speed Polyphase A-CMotor Drive Utilizing An In-Phase Current Signal For Motor Control,filed on May 28, 1971, in the names of Carlton E. Graf, Einar A.Skogsholm and Werner K. Volkmann and assigned to the assignee of thisinvention. Again, while the in-phase current signal is a preferredindicator of the starting of one or more motors in the motor bank andwhich a number of motors are already operating, it should be noted thatother parameters which change substantially during motor starting may bemonitored and represented by the magnitude of the signal on conductor38.

Referring again to FIG. 1, the control signal 38 is supplied to a changesensor which produces an output signal 102 only when the control signal38 increases substantially from its steady state level. In other words,an output signal 102 is produced only when there is a change in thein-phase current output on conductors 14 that is consistent with thestarting of one of the motors Ml-M8. Signal 102 is supplied to a voltageadjusting signal generator 104. In response to an output signal 102, thevoltage adjusting signal generator 104 produces the voltage increasingsignal 28. As explained previously, the voltage increasing signal 28 isproduced for a period of time sufficient to bring a motor being startedup to its synchronous speed. The signal 28 is then automaticallyremoved, and the output voltage of the inverter drops to a steady statelevel sufficient for motor operation.

FIG. 3 illustrates a preferred embodiment of the change sensor 100 andone embodiment of the voltage adjusting signal generator 104. The changesensor 100 includes an input resistor 106, a capacitor 108, and a diode110 connected in series between conductor 38 and conductor 102, thecathode of the diode 110 being connected to the conductor 102 and theanode of the diode 110 being connected to junction 112 and the capacitor108. A resistor 114 is connected between common 116 and the junction 112between the capacitor 108 and the diode 110.

If a number of the motors M1-M8 are operating under steady stateconditions, the control signal 38 will have a steady state levelproportional to the in-phase components of current supplied to themotors. Under these conditions, the capacitor 108 and the dischargeresistor 114 maintain the junction 112 at the potential of common 116,and no output signal is supplied to conductor 102. If the control signal38 should rise slightly, a corresponding increase will appeartransiently at junction 112, but not on conductor 102 since the voltagedrop on the diode 110 will be insufficient to cause conduction.Similarly, a substantial reduction in the control signal 38 will not betransmitted through the diode 110 due to the polarity of the diode.However, a substantial increase in the control signal 38 resulting fromthe starting of an additional motor will result in a correspondingtransient rise in the voltage at junction 112 and the transmission of acharge signal 102 to the voltage adjusting signal generator 104.

Still referring to FIG. 3, the voltage adjusting signal generator 104includes an input junction 120 coupled to receive the signal 102 and anoutput resistor 122 coupled between the junction 120 and conductor forvoltage signal 28. A capacitor 124 is connected between the junction 120and common 116, and a pair of diodes 126a and 126k are connected inseries between the junction 120 and common 116. The operation of thevoltage adjusting signal generator 104 will now be described. When achange signal is supplied to conductor 102 and input junction 120, theinput signal is transmitted through the resistor 122 as a voltageincreasing signal 28 to the regulator to increase the inverter outputvoltage, the diodes 126a and 126k limiting the magnitude of the signal28 to an acceptable level. During the presence of the signal onconductor 102, the capacitor 124 is charged to a voltage greater thanthat of common 116. Following the removal of the signal 102 from thechange sensor 100, the capacitor 124 discharges through the resistor 122to continue to supply a voltage increasing signal over conductor 28 fora period of time sufficient to assure synchronization of the motor beingstarted. Once the capacitor 124 is discharged, the signal 28 disappears,and the output voltage of the inverter will return to its lower steadystate level. Through proper selection of the resistor 122 and thecapacitor 124, the duration of the voltage boost can be controlled. Inone application where the in-phase current signal on conductor 38 rangedfrom a typical steady state range of 1 volt to 5 volts to a startingcurrent level in the range of 3 volts to 7 volts, it was found that avoltage boost signal 28 of approximately 0.05 milliamps was required forapproximately 2 seconds. This was accomplished by selecting the circuitcomponents as follows: resistor 106 1,000 ohms; capacitor 108 I00microfarads; resistor 114 25,000 ohms; resistor 122 10,000 ohms; andcapacitor 124 200 microfarads. To accomplish voltage boost signals ofdifferent magnitudes and durations, appropriate changes in thecharacteristics of the circuit components can be made.

Referring now to FIG. 4, a second embodiment 104' of the voltageadjusting signal generator is illustrated, the generator 104' includingan input junction coupled to the conductor 102 and to a variableresistor 122' connected between the junction 120' and a junction 130.The junction 120' is also coupled to common 1 16 through a capacitor 124and a pair of series connected diodes 126a and l26b. These elementsfunction in a manner identical to the elements with correspondingunprimed numerals in H6. 3, the adjustability of the resistor 122 makingit possible to selectively adjust the length of the voltage adjustingsignal supplied to conductor 28. In order to assure that the voltageadjusting signal 28 has a substantially constant magnitude throughoutthe boost period, an operational amplifier 132 having infinite gain iscoupled to junction 130, the output junction 134 of the amplifier 132being connected through a potentiometer 136 to common 116. A clampcircuit comprising a l0 volt zener 138 and a diode 140 in parallel areconnected across the junctions and 134 so as to maintain a substantiallyconstant output signal at output junction 134 so long as the inputsignal at input junction 130 has any significant level, i.e., throughoutthe boost level selected by the setting of the resistor 122'. The actualmagnitude of the signal on conductor 28 may be adjusted at any desiredlevel through appropriate adjustment of the slider arm of thepotentiometer 136.

From the foregoing, it will be seen that this invention providesimproved means for assuring rapid and reliable starting of synchronousmotors in drive systems in which the motors being started and a numberof previously synchronized motors are connected to a single source ofadjustable voltage electric power. Furthermore, the rapid and reliablestarting is accomplished in accordance with the invention withoutchanging the operating speeds of the previously synchronized motors,without causing excessive heating due to operation at excessive voltagelevels, and without requiring the use of expensive and complex controlcircuitry.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form, details, andapplication may be made therein without departing from the spirit andscope of the invention. Although the invention has been illustrated in apolyphase drive system, it is equally applicable to single phasesystems. Similarly, it will be obvious that the invention is applicableto adjustable voltage drive systems utilizing a-c power sources otherthan inverters. Accordingly, it is intended that all such modificationsand changes be included within the scope of the appended claims.

What is claimed as new and is desired to secure by Letters Patent of theUnited States is:

1. In an a-c drive system including a source of adjustable voltage a-celectric power and a plurality of synchronous motors coupled to thesource for receiving electric power therefrom, control means comprising:

first means coupled to the source for sensing a selected outputparameter thereof having a magnitude which varies substantially from itssteady state level during the starting of one or more of the motors andfor producing a first control signal having a magnitude representativeof the magnitude of the selected output parameter, second means coupledto said first means for receiving first control signals therefrom andproducing in response to a substantial change in a first control signala second control signal,

third means coupled to said second means for receiving second controlsignals therefrom and producing in response to a second control signal athird control signal having a predetermined duration following theinitial reception of the second control signal,

and fourth means coupled to the source for controlling the outputvoltage of the electric power supplied therefrom, said fourth means alsocoupled to said third means for receiving third control signalstherefrom and responding to a third control signal by changing thevoltage of the electric power supplied by the source throughout thepredetermined duration of the third control signal.

2. Control means as defined by claim 1 in which the selected outputparameter sensed by said first means is the component of output currentthat is in phase with the output voltage of the source.

3. Control means as defined by claim 1 in which the magnitude of thirdcontrol signals produced by said third means is independent of therespective magnitudes of the changes in the first control signal.

4. Control means as defined by claim 1 in which the selected outputparameter sensed by said first means increases during the starting ofone or more of the motors, in which said second means comprises meansfor producing second control signals in response to substantialincreases or decreases in the selected output parameter as representedby respective increases or decreases of first control signals, in whichsaid means comprises means for generating third control signals only inresponse to second control signals produced in response to increases infirst control signals, and in which said fourth means increases theoutput voltage of the source throughout the predetermined duration ofthird control signals, whereby the output voltage of the source isautomatically increased during the starting of one or more motors so asto implement rapid synchronization of the motors being started withoutchanging the operating speed of previously synchronized motors.

5. Control means as defined by claim 4 in which the selected outputparameter sensed by said first means is the component of output currentthat is in phase with the output voltage of the source.

6. Control means as defined by claim 4 in which said second meansincludes means for producing second control signals of differentpolarity in response to, respectively, increases and decreases in thefirst control signal, and in which said third means includesunidirectional input means selected so as to transmit only secondcontrol signals produced in response to increases in the first controlsignal.

7. Control means as defined by claim 6 in which the magnitude of thirdcontrol signals produced by said third means is independent of themagnitude of second control signals transmitted by said unidirectionalinput means.

8. Control means as defined by claim 7 in which said third means furthercomprises means for maintaining the magnitude of third control signalssubstantially constant throughout the predetermined duration.

9. Control means as defined by claim 7 in which said third means furthercomprises means for adjusting the predetermined duration of thirdcontrol signals and means for adjusting the magnitude of third controlsignals.

10. Control means as defined by claim 9 in which said third meansfurther comprises means for maintaining the magnitude of third controlsignals substantially constant throughout the predetermined duration.

11. An adjustable speed drive system comprising:

an inverter for supplying adjustable frequency, ad-

justable voltage electric power,

a plurality of synchronous motors coupled to said inverter for receivingelectric power therefrom, means for independently starting and stoppingsaid motors,

first means coupled to said inverter for sensing a selected outputparameter thereof which increases substantially from its steady statelevel during the starting of one or more of the motors and for producinga first control signal having a magnitude representative of themagnitude of the selected output parameter,

second means coupled to said first means for receiving first controlsignals therefrom and producing in response to a substantial change in afirst control signal a second control signal, third means coupled tosaid second means for receiving second control signals therefrom andproducing in response to a second control signal responsive to anincrease in a first control signal a third control signal having apredetermined duration following the initial reception of the secondcontrol signal,

and fourth means coupled to said inverter forcontrolling the outputfrequency and the output voltage of the electric power suppliedtherefrom, said fourth means also coupled to said third means forreceiving third control signals therefrom and responding to a thirdcontrol signal by increasing the output voltage of said inverterthroughout the predetermined duration of the third control signal,whereby the output voltage of said inverter is automatically increasedduring the starting of one or more motors so as to implement rapidsynchronization of the motors being started without changing theoperating speed of previously synchronized motors.

12. Control means as defined by claim 11 in which said second meansincludes means for producing second control signals of differentpolarity in response to, respectively, increases and decreases in thefirst control signal and in which said third means includesunidirectional input means selected so as to transmit only secondcontrol signals produced in response to increases in the first controlsignal. I

13. Control means as defined by claim 12 in which the magnitude of thirdcontrol signals produced by said third means is independent of themagnitude of second control signals transmitted by said unidirectionalinput means.

14. Control means as defined by claim 13 in which said third meansfurther comprises means for maintaining the magnitude of third controlsignals substantially constant throughout the predetermined duration.

15. Control means as defined by claim 14 in which said third meansfurther comprises means for adjusting the predetermined duration ofthird control signals and means for adjusting the magnitude of thirdcontrol signals.

16. Control means as defined by claim 15 in which the selected outputparameter sensed by said first means is the component of output currentthat is in phase with the output voltage of said inverter.

17. A method of providing rapid synchronization of one or moresynchronous motors being started in a system in which a source ofadjustable frequency, adjustable voltage electric power supplieselectric power to the motors being started and to a plurality ofpreviously operating motors, said method comprising:

sensing a selected output parameter of the source which changessubstantially from its steady state level during the starting of one ormore motors, and increasing the output voltage only of the source for apredetermined period following the sensing of a change in the selectedoutput parameter which is consistent with the starting of one or moremotors, whereby the increased output voltage implements rapidsynchronization of the motors being started without changing theoperating speed of the previously operating motors.

1. In an a-c drive system including a source of adjustable voltage a-celectric power and a plurality of synchronous motors coupled to thesource for receiving electric power therefrom, control means comprising:first means coupled to the source for sensing a selected outputparameter thereof having a magnitude which varies substantially from itssteady state level during the starting of one or more of the motors andfor producing a first control signal having a magnitude representativeof the magnitude of the selected output parameter, second means coupledto said first means for receiving first control signals therefrom andproducing in response to a substantial change in a first control signala second control signal, third means coupled to said second means forreceiving second control signals therefrom and producing in response toa second control signal a third control signal having a predeterminedduration following the initial reception of the second control signal,and fourth means coupled to the source for controlling the outputvoltage of the electric power supplied therefrom, said fourth means alsocoupled to said third means for receiving third control signalstherefrom and responding to a third control signal by changing thevoltage of the electric power supplied by the source throughout thepredetermined duration of the third control signal.
 2. Control means asdefined by claim 1 in which the selected output parameter sensed by saidfirst means is the component of output current that is in phase with theoutput voltage of the source.
 3. Control means as defined by claim 1 inwhich the magnitude of third control signals produced by said thirdmeans is independent of the respective magnitudes of the changes in thefirst control signal.
 4. Control means as defined by claim 1 in whichthe selected output parameter sensed by said first means increasesduring the starting of one or more of the motors, in which said secondmeans comprises means for producing second control signals in responseto substantial increases or decreases in the selected output parameteras represented by respective increases or decreases of first controlsignals, in which said means comprises means for generating thirdcontrol signals only in response to second control signals produced inresponse to increases in first control signals, and in which said fourthmeans increases the output voltage of the source throughout thepredetermined duration of third control signals, whereby the outputvoltage of the source is automatically increased during the starting ofonE or more motors so as to implement rapid synchronization of themotors being started without changing the operating speed of previouslysynchronized motors.
 5. Control means as defined by claim 4 in which theselected output parameter sensed by said first means is the component ofoutput current that is in phase with the output voltage of the source.6. Control means as defined by claim 4 in which said second meansincludes means for producing second control signals of differentpolarity in response to, respectively, increases and decreases in thefirst control signal, and in which said third means includesuni-directional input means selected so as to transmit only secondcontrol signals produced in response to increases in the first controlsignal.
 7. Control means as defined by claim 6 in which the magnitude ofthird control signals produced by said third means is independent of themagnitude of second control signals transmitted by said unidirectionalinput means.
 8. Control means as defined by claim 7 in which said thirdmeans further comprises means for maintaining the magnitude of thirdcontrol signals substantially constant throughout the predeterminedduration.
 9. Control means as defined by claim 7 in which said thirdmeans further comprises means for adjusting the predetermined durationof third control signals and means for adjusting the magnitude of thirdcontrol signals.
 10. Control means as defined by claim 9 in which saidthird means further comprises means for maintaining the magnitude ofthird control signals substantially constant throughout thepredetermined duration.
 11. An adjustable speed drive system comprising:an inverter for supplying adjustable frequency, adjustable voltageelectric power, a plurality of synchronous motors coupled to saidinverter for receiving electric power therefrom, means for independentlystarting and stopping said motors, first means coupled to said inverterfor sensing a selected output parameter thereof which increasessubstantially from its steady state level during the starting of one ormore of the motors and for producing a first control signal having amagnitude representative of the magnitude of the selected outputparameter, second means coupled to said first means for receiving firstcontrol signals therefrom and producing in response to a substantialchange in a first control signal a second control signal, third meanscoupled to said second means for receiving second control signalstherefrom and producing in response to a second control signalresponsive to an increase in a first control signal a third controlsignal having a predetermined duration following the initial receptionof the second control signal, and fourth means coupled to said inverterforcontrolling the output frequency and the output voltage of theelectric power supplied therefrom, said fourth means also coupled tosaid third means for receiving third control signals therefrom andresponding to a third control signal by increasing the output voltage ofsaid inverter throughout the predetermined duration of the third controlsignal, whereby the output voltage of said inverter is automaticallyincreased during the starting of one or more motors so as to implementrapid synchronization of the motors being started without changing theoperating speed of previously synchronized motors.
 12. Control means asdefined by claim 11 in which said second means includes means forproducing second control signals of different polarity in response to,respectively, increases and decreases in the first control signal and inwhich said third means includes unidirectional input means selected soas to transmit only second control signals produced in response toincreases in the first control signal.
 13. Control means as defined byclaim 12 in which the magnitude of third control signals produced bysaid third means is independent of the magnitude of second controlsignals transmitted by said unidirectional Input means.
 14. Controlmeans as defined by claim 13 in which said third means further comprisesmeans for maintaining the magnitude of third control signalssubstantially constant throughout the predetermined duration. 15.Control means as defined by claim 14 in which said third means furthercomprises means for adjusting the predetermined duration of thirdcontrol signals and means for adjusting the magnitude of third controlsignals.
 16. Control means as defined by claim 15 in which the selectedoutput parameter sensed by said first means is the component of outputcurrent that is in phase with the output voltage of said inverter.
 17. Amethod of providing rapid synchronization of one or more synchronousmotors being started in a system in which a source of adjustablefrequency, adjustable voltage electric power supplies electric power tothe motors being started and to a plurality of previously operatingmotors, said method comprising: sensing a selected output parameter ofthe source which changes substantially from its steady state levelduring the starting of one or more motors, and increasing the outputvoltage only of the source for a predetermined period following thesensing of a change in the selected output parameter which is consistentwith the starting of one or more motors, whereby the increased outputvoltage implements rapid synchronization of the motors being startedwithout changing the operating speed of the previously operating motors.